106 COLORADO FORMATION AND ITS INVERTEBRATE FAUNA, [bull. 106. 



surface-markings, but sometimes they present faint remains of concen- 

 tric undulations. The posterior umbonal ridges, however, are always 

 strongly defined, and angular even on internal casts. Some large speci- 

 mens have, in casts, a strong, obtuse ridge near the dorsal margin of 

 each valve, apparently bounding a kind of broad lanceolate escutcheon, 

 the middle and deeper part of which is occupied by the rather narrow 

 ligament. 



" Young specimens of this shell present much the general outline of 

 small individuals of Veniella ( Venilicardia obtruncata) Stoliczka, (Pal- 

 aeont. Ind., in, PL vni, Figs. 7 and 7«, b); but they have the umbonal 

 ridge of each valve much more deeply defined, while the hinge charac- 

 ters of the two shells are altogether different. Of course, if Dr. Mor- 

 ton's name Venilia should be retained for this genus, the name of this 

 species would become Venilia goniophora. 



u Locality and position. — Missouri river, near Fort Benton ; Fort Ben- 

 ton group of the Cretaceous." 



Dr. White reports the species from " southeast of Paria, Utah." l 



There is some reason for doubting that this is really distinct from 

 Veniella mortoni. 



VENERID^E. 



Genus TAPES Megerle v. Muhlfeldt. 



Tapes cyrimeriformis n. sp. 



PL xxiv, Figs. 1-G. 



Shell of medium size, equivalve, moderately convex, subcircular in 

 outline; beaks small, approximate, situated a little in advance of the 

 middle; cardinal margin sloping with nearly equal gentle convexity 

 in both directions from the beaks; anterior end broadly and regularly 

 rounded; posterior end slightly subangular above and rounded below; 

 basal margin forming a regular gentle curve. Surface marked by fine 

 lines of growth. There is no distinctly defined limule. The ligament, 

 which is preserved on some specimens, is relatively large and prominent. 



Hinge of right valve with three strong, divergent, cardinal teeth, of 

 which 5 the posterior is deeply bifid and the other two are smaller and 

 more approximate. In the left valve, which also has three teeth, the 

 anterior tooth is very small, the middle one is larger and slightly bifid, 

 and the posterior one is very long and oblique and situated near the 

 outer margin of the hinge. 



The muscular scars are large, ovate, not very strongly marked; pal- 

 lia! sinus deep, ascending and narrowly rounded at the extremity. 



One of the type specimens measures 49 mni in length, 42 n,m in height, 

 and 16 mm in convexity of both valves united. The corresponding meas- 



1 See U. S. Geog. and Geol. Sur. West 100th Meridian, vol. IV, p. 185. 



